Many promising therapeutics are increasingly protein-based;1-3 however, bioactive protein delivery remains a challenge.4 Two main approaches have emerged to control protein release: (i) encapsulation in nano-/micro-particles, which provides a diffusive barrier and (ii) incorporation in affinity-based drug delivery systems, which establishes a dynamic equilibrium to delay release.5,6 Although protein encapsulation is common, the harsh environments (organic solvents, aqueous/organic interfacial free energy, shear force, and lyophilization) present during the encapsulation process can diminish protein bioactivity and drug loading is generally low.7,8 Affinity-based release systems overcome these limitations by sequestering proteins, commonly growth factors, in a polymer or polymer matrix, much like the extracellular matrix in vivo. These systems generally consist of a polymer (naturally occurring or synthetic and degradable, bioresorbable or biostable), such as a hydrogel, that has been chemically modified to bind a growth factor with moderate or high affinity, depending on the required rate of release, to attenuate the diffusional release of the protein.6 For example, heparin or heparin-binding peptides have been immobilized to various matrices to deliver a variety of heparin-binding proteins;9-18 however, this approach is inherently limited to heparin-binding proteins. Recombinant human basic fibroblast growth factor (rhFGF2) binding peptide can be used to control the release of rhFGF2 from PEG hydrogels, yet is similarly limited to FGF2.19 Collagen scaffolds have been shown to bind therapeutic fusion proteins that contain a collagen binding domain;20 however, this system requires collagen as a scaffold and the rate of release cannot be tuned. A system which can deliver a diversity of therapeutic agents, including proteins, with a tunable rate of protein release is required.